Even if someone hasn't received an invite to Party Finder, they can now join your party via a link! Your guest will only be able to join your party after you accept them. Unfortunately for them, they still won't have access to any other parties, but for now you can play games together.
First, create your party.
Then click the Copy Link button and send that to your friends.
They will then be directed to create their own account or sign into Parsec. Once they do that, they can request to join the party with a nice message. As you can see, this new guest doesn't have access to Party Finder yet since they're being asked to join the invite list, but they can access this particular party :).
Your guest will wait until you accept his request to join.
Once you accept the request to join, the guest can now request to Co-Play!